Conventionally, accessing a subsea well (e.g., for production therefrom and/or performing various operations on or within the wellbore) requires use of a conduit, known as a riser, which extends from the wellhead of the subsea well to or near the surface of a body of water. While the specific structure and features of risers can vary, in general, each riser will include a number of steel tubular segments, threaded or otherwise connected to one another, to span the distance between the subsea wellhead and the surface. Due to the significant length of a riser, it is expected that various forces, such as heave, wave motion, currents, and/or other similar forces imparted by the body of water, impacts with subsea objects, and/or the weight and flexibility/sway of the riser itself, will cause the riser to move and/or bend to a certain extent. Additionally, wind forces applied to a surface object, such as a semisubmersible or vessel engaged to the upper end of the riser, and/or movement of the surface object, can also impart a force to the riser.
Due to the limited flexibility of the steel segments of a riser, special measures must be taken to compensate for forces that could otherwise flex or move a riser beyond its structural integrity, causing the riser to become damaged. For example, some types of motion (e.g., heave forces) experienced by risers and/or surface objects engaged thereto can be compensated for using various cylinder-based compensation systems that cause the riser and/or other objects to remain effectively stationary relative to other objects and/or to the Earth's surface. However, in nearly all cases, at least some lateral motion and/or bending will be experienced by all portions of the riser, to some extent, e.g., a lateral movement of the upper end of the riser will cause the lowest point of the riser to bend slightly to account for this movement, the difference between the relative movements of the upper and lower ends depending on the total length of the riser.
To allow for this expected bending motion most riser systems include a stress joint secured at the base of the riser. Conventional stress joints are unique structures, each specifically and precisely engineered to account for the forces and movements expected to be experienced by a riser, based on the riser length, thickness, materials, depth, and various meteorological and oceanographic (metocean) environments. Thus, a custom-designed stress joint is normally designed and constructed for each specific subsea well and riser condition. A typical stress joint is a tapered structure, wider at its base than its upper end, the taper angles and radii of curvature along the body of the joint being precisely designed to allow a certain amount of bending commensurate with the expected motion of the upper end of the riser. While a stress joint is normally secured, to a subsea wellhead at its lower end, and to a riser at its upper end, substantially similar structures are usable in other positions and/or applications. For example, a keel joint can be secured at the upper end of a riser, the keel joint having a structure substantially similar or identical to that of a stress joint, but inverted, e.g., a typical keel joint has a tapered body with a wide end oriented to face upward, while a narrower end, facing downward, engages the upper end of the riser. Stress joints are also sometimes used at curved points along a riser (e.g., a catenary joint.)
Most stress joints are formed from steel, and must be a single-piece, unitary structure due to the fact that a multiple-part structure would be subject to weaknesses and additional forces at the points of engagement between parts. As a result, stress joints are an extremely expensive part of a riser system, both due to the unique design engineering involved, the massive, precision construction thereof, as well as the difficulties and costs inherent in qualifying, testing, and transporting the single-piece, heavy structure to a subsea location. Extensive time and expense is required when custom designing and manufacturing each stress joint for each specific condition and/or configuration. Under some circumstances, the length of a riser and/or the expected movement thereof or forces applied thereto render use of a unitary steel stress joint impossible due to the fact that a stress joint able to account for the expected forces and motion would be prohibitively large, and nearly impossible to construct or transport. In such cases, other, more flexible materials, such as titanium, have been used to form stress joints. Existing titanium stress joints must still be precisely engineered based on the specific features of each unique well and riser, and still include tapered, one-piece bodies, and as such, remain costly and cumbersome items, due not only to construction and transport difficulties and costs, but also due to the increased cost of the materials when compared to steel. Additionally, titanium stress joints include welded flanges, which create points of stress, weakness, and/or unfavorable distribution of forces that must be accounted for during the design and engineering process. Furthermore, much like their steel counterparts, titanium stress joints also require extensive time and expense to design and manufacture.
A need exists for stress joints that are adjustable (e.g., modular), thus able to be used with a variety of subsea well and riser configurations, and able to be recovered after use and reused with other wells and risers.
A need also exists for stress joints that incorporate combinations of parts and materials that effectively compensate for the forces applied to a riser, while remaining low in cost, reliable, and convenient to construct and transport when compared to large, single-piece structures.
A further need exists for stress joints that can be available for use rapidly, such as through immediate transport and installation of pre-manufactured and stored parts usable with a large variety of subsea well and riser configurations.
Embodiments usable within the scope of the present disclosure meet these needs.